To increase the performance of inkjet printers, one pass bi-directional printing may be employed. This means that ink is ejected in both directions when the inkjet printheads are moving rom left to right across a swath of media, and also when they are moving from right to left across a swath of media. Bi-directional printing is distinguished from single-direction printing, in which ink is ejected when the inkjet printheads are moving from either left to right or from right to left across a media swath, and not in both directions. Using bi-directional printing can typically increase the performance of inkjet printers by roughly a factor or two.
For color printers, a color space conversion is usually performed on an image prior to its being printed. A color space is a mathematical model that is used to represent the colors of an image. By combining the corresponding constituent colors of a color space in different ways, any desired color can be represented. The typical color space for display purposes is red-green-blue (RGB), whereas a type of color space used for printing purposes is cyan-magenta-yellow (CMY), or cyan-magenta-yellow-black (CMYK). Depending on the color space being used, combining the constituent colors of the model produces different visual results.
Color space conversion, such as from an RGB color space to a CMY, CMYK, or other color space, typically employs a three-dimensional table, called a color map. For each unique combination of red, green, and blue values, the map provides corresponding values for cyan, magenta, yellow, and black. Once all the pixels of an image are converted from RGB to a given printer's resident color model, printing may occur.
To avoid visual artifacts, such as hue shift, when using bi-directional color inkjet printers, color space conversion can be performed for each direction in which the inkjet printheads travel over the media. For instance, there may be a first color map that translates , G, B values in the RGB color space to C, M, Y, K values in the CMYK color space that is used when the inkjet printheads travel from left to right over media swaths. There may also be a second color map that translates the same R, G, B values in the RGB color space to C, M, Y, K values in the CMYK color space that is used when the printheads travel from right to left over the media swaths.
Usually, such bi-directional color maps are constructed offline, under nominal conditions for a given color inkjet printer. These nominal conditions include a nominal printhead being used with ideal ink drop weights, ideal inks, particular types of target media, as well as standard operating temperature, humidity and other environmental factors. In real-world applications, however, the conditions in which the printer is used may depart from the nominal conditions in which the bi-directional color maps were developed. This can affect the accuracy of the bi-directional color maps, affect printer output quality, and affect bi-directional printing robustness. For instance, some banding artifacts may become visible, mainly due to the color hue shift between the swaths printed by the printheads traveling from left to right and the swaths printed by the printheads traveling from right to left.